Couplers are optical elements that join a light source, such as a laser diode, a light-emitting diode, an optical fiber, or other light source, to a light collector, such as a photo detector, an optical fiber, an optoelectronic chip, an optical fiber, and the like.
Prior art couplers comprise either (1) homogeneous glass, or plastic lenses with either spherical, aspherical or cylindrical surfaces, (2) optical fibers and optical fiber based cylindrical lenses, (3) diffractive optic elements, (4) Fresnel lenses, or (5) radially-graded index of refraction glass cylinders or rods. The homogeneous glass, optical fiber based cylindrical, or plastic lenses require air space and have a low numerical aperture or light collection value, which results in less light being delivered to the collector than was incident on the coupling lens. In addition, homogeneous glass or plastic lenses are incapable of correcting for spherical aberration, which results in inefficient coupling, unless an aspheric surface is formed on the lens. Finally, multiple elements are often required in order to sufficiently couple the light.
The radially-graded index glass cylinder lenses when functioning with flat surfaces and focusing light at the lens exit surface, overcome one of the problems of homogeneous glass lenses. Further, the radially-graded index glass cylinder lenses comprise a single optical component, thereby overcoming another problem of homogeneous glass lenses. However, these radially-graded index glass cylinder lenses are incapable of correcting spherical aberrations or of forming small focal points, resulting in inefficient coupling performance. While coupling performance may be improved by forming a curved surface on the exit surface of the lens, the advantage of a flat surface is lost, and an air space is introduced into the coupling scheme. Radially graded index cylinder lenses when coupling light through their sides rather than their ends function with an air space requirement and lose the advantage of focusing light to a flat surface. Also, radially graded index lenses with flat surfaces lack sufficient batch to batch manufacturing reproducibility such that lenses performance varies so significantly that their effective focal length often varies too much to satisfy the strict manufacturing tolerance specifications necessary for many devices to be efficiently mass produced without coupling airspace. Additionally, radially graded index coupling lenses with curved surfaces have the disadvantage of requiring a varying degree of surface curvature in order to minimize the inherent spherical aberration introduced by the less than ideal profile of index change in these lenses that is limited by their manufacturing process requirements. Finally, radially graded index of refraction coupling lenses are limited in the amount of index change that can be placed into these lenses, which is several times less than what can be placed into the axially graded index lenses that are central to the invention to be described hereunder. Diffractive optical elements suffer from requiring long focal lengths and poor transmission efficiency when coupling a plurality of different wave-lengths. Fresnel lenses suffer from poor image quality and long focal length.
Solar concentrators are used as a conduit to direct incident sun light onto one or more solar cells, such as silicon-based solar cells or III-V multi-junction solar cells. The higher the concentration ratio, the smaller the solar concentrator and the higher the power output per volume or weight.
Solar concentrators employing gradient refractive index material are disclosed in U.S. Pat. Nos. 4,883,522, issued Nov. 28, 1989, and 4,907,864, issued Mar. 13, 1990. While these patents certainly represented an advance in the art of solar concentrators, efforts to further improve such solar concentrators have been on-going.